Null hypothesis: There is limited or no selection on the polymorphisms of Cepaea nemoralis and all variation can be explained by genetic drift and gene flow
Alternative hypothesis: There is significant selection on the polymorphisms of Cepaea nemoralis and only some of the variation can be explained through the action of genetic drift and gene flow.
If the null hypothesis is incorrect then selection is having a greater impact on the ratios of polymorphisms in the samples compared to genetic drift and gene flow. For example, the proportion of Cepaea nemoralis with more stripes/darker colouration could be greater in the woodland area than the grassland area in all samples; the reverse could be true for Cepaea nemoralis with lighter colouration.
If the null hypothesis is correct most of the polymorphism ratio will be due to genetic drift and gene flow. As a result the ratio of banded to unbanded Cepaea nemoralis will not have clear correlation between samples in the same environments. If genetic drift has a predominant effect on the ratios then each polymorphism could appear in a high ratio at random in samples from the same environment. If gene flow has a predominant effect then the ratios would be relatively even across all the samples.
If sampling error occurs the ratios of polymorphisms would be similar to those of genetic drift. However the Chi squared test should rule this type of error out.
Human error could be ruled out by comparing results with other groups that have the same sampling strategy; however the study hopes to prevent human error through accurate unbiased sampling.
Sampling Strategy
Sample from 2 different habitats: 3 in grassland and 3 in woodland
Samples taken from the same altitude
4x4 metre square for each sample
Samples must be 20 metres apart measured from the edge of the 4x4 metre square
No samples will be taken from any ground that is regularly used by humans, for instance foot paths
If possible measure along the side of woodland that receives the most sunlight (decreased incidence of shadows on grassland) which will reduce the chances of gene flow as it is assumed that there is a selection pressure on shell polymorphisms based on the amount of sunlight
Strengths
Samples will only be taken from 2 different habitats
Due to the low sample size introducing other variables will complicate the study
Sampling the habitat of shrubs will not be included
Clear difference in the habitats of grass and woodland based on light levels
This will show clear differences in shell polymorphisms because it is assumed in the study that there will be different selection pressures in different light levels. This will also clearly highlight genetic drift if it is found that one sample is far different from the others
Shrubs are isolated within grassland and have similar light levels; this means there is a much higher chance of gene flow which is not going to be tested in this study
Sampling along the same altitude
This eliminates another variable from complicating the study
Sampling technique
The 4x4 metre square for each sample will ensure maximum numbers of Cepaea nemoralis caught in the study which will increase the accuracy of conclusions
The samples are 20 metres apart which will reduce the risk of gene flow between populations and ensure the samples are as independent of one another as possible
Measuring in woodland will increase the amount of Cepaea nemoralis caught
Woodland is generally much moister than grassland, increasing the chances of catching more Cepaea nemoralis
Weaknesses
Number of samples is very limited
The study would benefit from pairing with another group to increase the number of samples from the specific altitude thereby increasing the accuracy of the study. However too many samples would increase the risk of comparing populations that undergo gene flow
The area of sampling is very small
A number of studies investigating Cepaea nemoralis have measured many hundreds of metres to determine an overall correlation. The small local population over a potential maximum of 100m is very small.
The number of variables is small
In an ideal world increase number of variables, such as measuring shrubs and also measuring the boundaries of different habitats to compare and be able to recognise gene flow.
Limit on space
In the area provided there may not be enough room to have each sample at 20metres apart. If this is the case the distance between samples will be reduced
Distinguishing between genetic drift, selection and gene flow
In an ideal situation Selection: The same ratios of shell polymorphisms between all samples of differing populations e.g. 80% heavily banded Cepaea nemoralis in woodland and 80% lightly coloured Cepaea nemoralis in grassland.
Gene flow: The same ratio of shell polymorphisms in adjacent samples on a boundary of environments e.g. 50:50 dark to light colouration in both woodland and grassland. Note: The study hopes to eliminate gene flow between the samples.
Genetic drift: Different ratio of shell polymorphisms in a sample compared to the other two from the same environment. For example, a very high ratio of dark banded Cepaea nemoralis in one sample of grassland would go against the assumed selection pressures and have a high likelihood of being due to genetic drift.
What is expected It is expected that all three of these processes will have an influence on all of the samples, producing results that are difficult to distinguish if gene flow or selection is having an effect. It could well be that all of the ratios in the samples are mainly due to genetic drift.
Introduction
Null hypothesis: There is limited or no selection on the polymorphisms of Cepaea nemoralis and all variation can be explained by genetic drift and gene flow
Alternative hypothesis: There is significant selection on the polymorphisms of Cepaea nemoralis and only some of the variation can be explained through the action of genetic drift and gene flow.
If the null hypothesis is incorrect then selection is having a greater impact on the ratios of polymorphisms in the samples compared to genetic drift and gene flow. For example, the proportion of Cepaea nemoralis with more stripes/darker colouration could be greater in the woodland area than the grassland area in all samples; the reverse could be true for Cepaea nemoralis with lighter colouration.
If the null hypothesis is correct most of the polymorphism ratio will be due to genetic drift and gene flow. As a result the ratio of banded to unbanded Cepaea nemoralis will not have clear correlation between samples in the same environments. If genetic drift has a predominant effect on the ratios then each polymorphism could appear in a high ratio at random in samples from the same environment. If gene flow has a predominant effect then the ratios would be relatively even across all the samples.
If sampling error occurs the ratios of polymorphisms would be similar to those of genetic drift. However the Chi squared test should rule this type of error out.
Human error could be ruled out by comparing results with other groups that have the same sampling strategy; however the study hopes to prevent human error through accurate unbiased sampling.
Sampling Strategy
Strengths
Weaknesses
Distinguishing between genetic drift, selection and gene flow
In an ideal situation
Selection: The same ratios of shell polymorphisms between all samples of differing populations e.g. 80% heavily banded Cepaea nemoralis in woodland and 80% lightly coloured Cepaea nemoralis in grassland.
Gene flow: The same ratio of shell polymorphisms in adjacent samples on a boundary of environments e.g. 50:50 dark to light colouration in both woodland and grassland. Note: The study hopes to eliminate gene flow between the samples.
Genetic drift: Different ratio of shell polymorphisms in a sample compared to the other two from the same environment. For example, a very high ratio of dark banded Cepaea nemoralis in one sample of grassland would go against the assumed selection pressures and have a high likelihood of being due to genetic drift.
What is expected
It is expected that all three of these processes will have an influence on all of the samples, producing results that are difficult to distinguish if gene flow or selection is having an effect. It could well be that all of the ratios in the samples are mainly due to genetic drift.